Missing materials or replacing materials
You will probably encounter demo formulations with some ingredients used, which you don't happen to have at hand. That's not a problem like trying to bake a cake without flour. Some ingredients can be left out entirely without doing to much damage to the concept. Or can be replaced with other "smell alike" ingredients. Always remember: small dosages won't necessarily mean less impact. Where leaving out 1 drop off huge-impact materials like ambrocenide or civet at 1% can have an enormous impact, while leaving out the full amount of benzyl alcohol can have little impact. And look-a-likes won't necessarily act or blend the same way. Substituting sugar in a cake with a mixture of honey and corn syrup seems like replacing "sweet" for "sweet", but the taste of the cake will be rather different. It's undoable to define replacements for all materials and often one needs to construct a replacement from many other ingredients. My starting point is always The Good Scents Company, they have an excellent odour search engine and odour profiles and molecular weight of thousands of ingredients. For example, looking for something that replaces "fresh green" can be found this way. Clicking on, for example, Galbanum, gives a tremendous amount of information, especially under the tabs organoleptics, properties and last but not least blenders!